Alkaline storage batteries such as nickel-cadmium batteries and nickel-metal hydride batteries are used in various applications due to having high capacity. Particularly, in recent years, alkaline storage batteries are envisioned as being used as, for example, a main power source for hybrid vehicles and electronic devices such as portable devices, and a backup power source for uninterruptible power systems. When used as above, the batteries are required to be charged in a short period of time, and over a wide range of temperatures including high temperatures. Thus, in charging the batteries over a wide range of temperatures, a high charge efficiency is required.
In an alkaline storage battery, a nickel oxide including, for example, nickel oxyhydroxide and nickel hydroxide is mainly used as the positive electrode active material. Moreover, as shown in the formula below, during charge, nickel hydroxide is converted into nickel oxyhydroxide, and during discharge, nickel oxyhydroxide is converted into nickel hydroxide. A positive electrode densely filled with such a nickel oxide has been proposed for use in an alkaline storage battery, in view of increasing capacity and output of the battery.
[Chem. 1]
Negative electrode: MH+OH−M+H2O+e−
Positive electrode: NiOOH+H2O+e−Ni(OH)2+OH−
Overall: NiOOH+MHNi(OH)2+M
(Here, M represents a hydrogen storage alloy.)
Moreover, in view of improving charge efficiency and capacity at a high temperature, a study is being made on adding a compound including, for example, Y, In, Sb, Ba, Ca, and Be to the positive electrode of an alkaline storage battery (c.f., Patent Literature 1). In view of improving active material utilization rate at a high temperature and cycle life, a proposal has been made to use 2 or more selected from the group consisting of Y, Yb, and Er as a compound (c.f., Patent Literature 2).
Furthermore, for the purpose of improving charge efficiency at a high temperature, a proposal has been made to make the amount of the sulfate radical (amount of SO42−) in the Ni(OH)2 crystal, 0.4 weight % or less (c.f., Patent Literature 3).